1. Field of the Invention
The present invention relates to medical diagnostic equipment, and more particularly to medical diagnostic equipment using radio-frequency (RF) fields, ultrasonic detectors and signal processors, to produce visual images and digital outputs.
2. Description of Related Art
Radio frequency (RF) electromagnetic (EM) fields are widely used for medical diagnostics in magnetic resonance imaging (MRI). In MRI, RF is used to excite coherent precession of nuclear magnetic moments in a strong static background magnetic field. Precessing moments provide imaging information through the subsequent RF fields that they generate. These subsequent fields depend on the local densities of moments, their relaxation times, and precession frequencies, which in turn depend upon the local chemical and physical environment of the precessing nuclei. Detection of the subsequent fields has provided images of molecular properties with good spatial resolution (to several micrometers). An imaging apparatus which relies on conventional MRI is expensive, bulky, lacks portability, and is associated with high facility costs because of the shielding and cryogenic requirements for strong magnetic fields.
Ultrasound, which usually produces images with millimeter resolution based on acoustic impedance differences between organs, has been used extensively for imaging in both medical and non-medical applications. Medical applications of ultrasound include imaging and flow measurements. Conventional ultrasound scanners are relatively inexpensive compared to MRI and have the advantage of portability.
Ultrasound has been used in conjunction with MRI to characterize the structure and quality of tissue imaged by magnetic resonance. MRI has also been used to sense organ motion produced by sound waves transmitted into the patient which gives a measure of the elasticity of tissues observed.
Another approach to diagnostic imaging requires application of electrical voltage to electrodes on the patient in a very strong steady magnetic field. Images are formed by detection of the resulting ultrasound. Alternatively, intense ultrasound is transmitted and the resulting voltages on the electrodes detected to form an image.
Microwave EM imaging without magnetic resonance has been considered for medical diagnostics. Excellent image contrast is produced because of the large variation of microwave EM refractive index among soft body tissues. However these large variations deflect the paths of the microwaves traversing the body and thus distort the image produced. This effect also causes variable concentrations of microwave power deposition in organs.
Direct application of electrical currents to the patient with observation of resulting voltages has been studied as an imaging method. Development and applications of these methods are motivated by the findings that malignant (cancerous) tissue may be differentiated from benign tissue by electrical conductivity. For example, breast cancer tumors have been reported to have electrical conductivity twenty to fifty times that of normal surrounding tissue. However, imaging of tissue conductivity from such measurements involves inverting an elliptic differential equation, which smoothes out source details at distant observation points. This process is unstable in the sense that a small change (e.g. due to noise or interference) in the received observational data produces large changes in the computed image.
In lieu of the application of electrodes to the body, alternating magnetic fields have been used to produce images from currents in a phantom and a human thorax during respiration. A problem with magnetic detection at sub-microwave frequencies is its inherently low resolution.
Other innovations recently introduced require such extensive data collection and processing that it must be done offline, taking significantly longer than the patient scan.